A dual parallel I/Q (in phase/quadrature) modulator typically has the structure schematically shown in FIG. 1. The signal enters a splitter 101, which divides the signal into each channel of four Mach-Zehnder modulators 110, each of which comprises two waveguides 111, and an electrical RF (radio frequency) transmission line 112. The modulators apply the required modulation, and the signal is recombined by a combiner 102.
The Mach-Zehnder modulator has a cross section along A as shown in FIG. 2. The waveguide section of the Mach-Zehnder modulator comprises a substrate 201, which contains waveguides 202. The RF transmission line array 203 is placed on one face of the substrate. Each RF transmission line comprises a signal electrode 204. Each signal electrode 204 has a ground electrode 205 on either side. The modulators may either be arranged x-cut (210), with the waveguides positioned below the gaps between the signal and ground electrodes (symmetrically around the signal electrode), or z-cut (220) with one waveguide beneath the signal electrode, and one beneath one of the ground electrodes. Normally, all modulators in an array would be the same type, but both z- and x-cut modulators are shown in FIG. 2 for illustration. Intermediate ground electrodes (i.e. those between two signal electrodes) are shared between adjacent RF transmission lines.